In recent years, a number of motors have been often used in an individual audio & video apparatus or an individual office automation apparatus. Some home appliances such as a dishwasher or a washing and drying machine also employs a plurality of motors.
Each one of the motors has been provided with a driving circuit and a control circuit so that the plurality of motors can be driven or controlled. The entire circuit thus becomes complicated, which prevents the apparatuses from being downsized or less expensive. To overcome this problem, it has been proposed that a plurality of driving circuits is integrated into one chip IC. This proposal is disclosed in, e.g. Japanese Patent No. 2662397.
The technique disclosed in this patent publication is this: A constant-current driving element, which incurs a greater loss among others, is disposed outside an IC chip so that an obstruction to integrating a plurality of driving circuits into one chip can be lessened, where the obstruction is an increase in heat generated due to greater loss in the IC chip.
FIG. 7 shows the related art disclosed in the foregoing patent publication. In FIG. 7, driving control IC 401 is formed of one chip containing a plurality of push-pull driving block 402, constant-current driving circuit block 403, and power-supply control block 404.
Push-pull driving block 402 outputs a bipolar output voltage, in response to a signal supplied to its input terminal, to DC motors M1 and M2 disposed outside of IC 401 via the IC's output terminals +OUT and −OUT. Constant-current driving circuit block 403 drives, in a constant-current manner, a load coupled to the output terminal in response to the signal supplied to the input terminal. An output from constant-current driving circuit block 403 drives, in a constant-current manner, solenoid S working as a plunger via driving transistor Tr1 externally coupled to driver IC 401. The foregoing structure allows lessening the loss inside the IC and achieving an integration of multiple driving control blocks into one chip IC.
Unexamined Japanese Patent Publication No. 2003-202719 discloses another technique independent of the forgoing technique disclosed in the patent publication. This one refers to a driving device which independently drives a plurality of DC brush-less motors so that a plurality of photosensitive drums of a tandem multi-color image-forming apparatus can be rotated. This invention proposes a technique of integrating the driving circuits of respective brush-less motors into one unit.
This technique allows sharing a part of functions except a switching section with the shafts of the plurality of motors, each one of which motors independently drives a plurality of photosensitive drums, and allows integrating the part of the functions into one chip for downsizing the motor and reducing the cost thereof. This switching section varies the outputs of the motors by switching over exciting coils.
A circuit operation of a conventional driving device of the motors disclosed in this publication is described hereinafter with reference to FIGS. 8 and 9. FIG. 8 shows a conventional circuit structure of this related art. FIG. 9 shows an internal structure of a conventional driving section.
The driving device shown in FIG. 8 drives and controls the objectives to be driven of the image forming apparatus, and is formed of driver 505, image-forming circuit drive device 500, and other motors 503, 504. Circuit drive device 500 is formed of four rotation drive devices. Each one of the rotation drive devices works as a driving source which supplies torque to each one of photosensitive drums 502A, 502B, 502C and 502D for driving those drums. The respective rotation drive devices are formed of sensor-less DC brushless motors 501A, 501B, 501C and 501D respectively, and speed reduction mechanisms corresponding to those sensor-less DC brushless motors, and encoders also corresponding to those motors.
Driver 505 receives a signal from the encoder for driving DC brushless motors 501A, 501B, 50C and 501D, and also drives other motors 503 and 504. This driver 505 is generally formed of various ICs mounted on a sheet of printed circuit board.
FIG. 9 shows an internal structure of driver 505 shown in FIG. 8. Driver 505 is formed of driving circuit 508, control IC 509, and back-electromotive voltage detecting circuit 510. Circuit 510 detects a back-electromotive voltage induced at each one of the phases of stator wirings of DC brushless motor 501A having a three-phase connection, and outputs the detection signal to driver IC 506. Driving circuit 508 is formed of one driver IC 506 and switching section (switching element) 507 responsive to the four shafts of the motors.
Driving circuit 508 receives a speed control signal from control IC 509, and then supplies a three-phase voltage driving signal to respective DC brushless motors 501A, 501B, 501C and 501D. As discussed above, at least a part of the functions except switching section 507 of driving circuit 508 is integrated into one chip, and this part of the functions is shared with the plurality of shafts of the motors. This function allows driver 505 to be downsized and less expensive.
Those related art discussed above need a driving transistor or a switching section to be connected externally to one chip IC, so that a further simplification, downsizing, or cost reduction of the circuit needs integration of all the elements of the circuit that drives the motor into one chip IC. However, a temperature rise due to the loss in the one chip IC should be attentively dealt with. It has been thus difficult to integrate all the elements into one chip IC.
Meanwhile, a CD auto-changer for instance employs motors as driving sources for mounting/removing a disc, lifting/lowering a disc tray, and rotating a disc; however, those motors do not need to work together. Some of the apparatuses having a plurality of motors work in a similar way to the CD auto-changer, i.e. those motors also do not need to work together. Those some of the apparatuses are disclosed in PCT international publication number WO2002/086883. Here is another instance; a household dishwasher is equipped with a plurality of motors including a washing motor, discharging pump motor, and fan motor, and these motors work alternately. This instance is disclosed in, e.g. Unexamined Japanese Patent Publication No. 2001-286175.
A technique is thus proposed for switching over a motor to be driven in an apparatus that employs a plurality of motors and makes those motors work alternately. The proposed technique makes a plurality of motors to be driven by a driving circuit change over to each other and work alternately in the apparatus equipped with the motors that do not need to work together. The technique thus allows the motor driving circuit and its control circuit to be shared with the plurality of motors, and achieves the downsizing and the cost reduction of the apparatus.
FIG. 10 shows the related art disclosed in the foregoing Unexamined Japanese Patent Publication No. 2001-286175. In FIG. 10, inverter circuit 703 converts a dc power of rectifying circuit 702 connected to ac power supply 701 into an ac power, and an output from inverter circuit 703 is switched over by load switcher 704 for driving a plurality of motors 705A, 705B sequentially. This structure allows unifying inverter circuit 703 with control circuit 706 of motors 705A, 705B into one body, so that the unified body can be shared with motors 705A, 705B.
However, this related art has the following problem. Circuit operation of the motor drive device disclosed in the foregoing Unexamined Japanese Patent Publication No. 2001-286175 is described hereinafter with reference to FIGS. 10-12. FIG. 11 shows a timing chart in switching over the motors being driven by the conventional motor drive device based on the related art. The vertical axis represents waveforms of a work/stop switching signal, a motor switching signal, and a surge voltage at the switching. The horizontal axis represents a time. FIG. 12 shows a timing chart in switching over the motors with the conventional motor drive device, and its vertical axis represents waveforms of work/stop switching signal, a motor switching signal, an rpm of a first motor, and an rpm of a second motor. The horizontal axis represents a time as that of FIG. 11 does.
According to the foregoing Unexamined Japanese Patent Publication No. 2001-286175, the motor drive device allows the motors to share inverter circuit 703 driving a plurality of motors and control circuit 706, and motors 705A, 705B are switched over to each other by load switcher 704. However, a stop of inverter circuit 703 generates back-electromotive force due to inertia rotation of the first motor, so that surge voltage 804 is generated at a power switching element of inverter circuit 703. Load switcher 704 needs to directly switch over the lines on which a large current runs, so that it is difficult to integrate load switcher 704 with other elements into one chip IC. Switcher 704 is thus obliged to externally connect to the IC via a mechanical relay. Use of the mechanical relay in order to switch over inverter circuit 703 invites an arc discharge due to the surge voltage, so that a contact life of the relay becomes shorter and the reliability of the relay lowers. To overcome this problem, a given delay time 905 should be prepared, and then load switcher 704 is operated, which obliges the switching time to be longer.